1. Field of the Invention
The present invention relates in general to a method and apparatus for cleaning surfaces, and in particular to the removal of organic films, particulate matter and other contaminants from the surface of semiconductor wafers.
2. Description of the Prior Art
The removal of contaminants from surfaces is critical for the profitable manufacturing and subsequent performance of many devices and processes. For example, device yields in semiconductor fabrication facilities are adversely affected by defects caused by particulates adhering to wafer surfaces. More than 80% of the yield loss of volume-manufactured VLSI's is attributed to particulate microcontamination. As device geometries continue to shrink and wafer sizes increase, particle contamination will have an ever increasing impact on device yields. New technologies will be required to clean wafer surfaces to meet national goals for producing 0.07 micron feature sizes by the year 2010. It is now recognized that the future need in semiconductor wafer processing requires removal of particulates 0.1 micron in size and smaller which are highly resistant to removal by conventional cleaning technologies. Present particle removal technologies become increasingly ineffective as "killer" particle size decrease.
Particles generated within process tool equipment, especially in the backend of a multilevel process, represent a major source of yield loss in terms of defective chips. At the present time, there is no commercially available, in-situ cleaning instrumentation for processing wafers in vacuum. The requirements placed on surface cleanliness for microelectronics device fabrication also apply to the manufacturing of micromachines and microsensors based on silicon or gallium arsenide wafer preparation technology.
In addition to cleaning semiconductor wafers and processing tools, the present invention also relates to the cleaning of ground or spacecraft optics such as mirrors, lenses and windows. Other areas of application of the invention include the cleaning of silicon or other substrate materials to lower costs and uphold reliability during the manufacturing of flat panel displays; cleaning spacecraft thermal control surfaces and solar panels; cleaning surfaces in preparedness for deposition of thick or thin film materials to improve adhesion or growth dynamics; precision cleaning and removal of contaminants from vacuum chamber walls and internal mechanical/optical systems in major facilities such as the National Ignition Facility for fusion research and surfaces critical for the control of pharmacological cross-contamination.
Additional areas of application for the present invention include the cleaning of critical surfaces relevant to computers such as magnetic disk storage media. The continued evolution of computer technology has resulted in increasing demands for chemically clean and particulate-free surfaces. As computer technology continues to rely on microelectronics devices that shrink in size, product yield has become increasingly vulnerable to chemical and particulate contaminants.
Thin film structures are used in a variety of industrial applications including optical components, industrial platings, solar cells, wear and corrosion resistant coatings and coatings for transmissive and reflective elements to name a few. Thin films structures are adversely affected by the presence of chemical and micron-sized contaminants which impede the growth, adhesion, wear resistance and stability of the films. The present invention provides an enhanced cleaning process for a variety of solid surfaces compared to conventional cleaning techniques used in the above applications. For a review of cleaning techniques for removing particulates from surfaces, see J. Bardina, "Methods For Surface Particle Removal: A Comparative Study", Particulate Sci.Technol., 6, 121, 1988.
At present there are two principal methods of cleaning wafer surfaces: liquid phase or "wet" cleaning and gas phase or "dry" cleaning designed to remove process chemicals, films and particulate contamination. These methodologies suffer from several drawbacks, the most serious being that no single technology rids surfaces of organic films, trace metallic elements or particulates simultaneously. In some cases, the cleaning process is a source of contaminants itself. Even megasonic techniques, which can remove particulates in a give size range may not be effective for removing particulates &lt;0.1 micron. Furthermore, ultrasonic cleaning efficiencies show some dependency on particulate composition and morphology. Wet cleaning technologies also suffer by consuming large quantities of water. The need to conserve water and reduce costs associated with water usage are obvious. Additionally, wet cleaning technologies consume large quantities of environmentally hazardous chemicals such as inorganic acids, bases and etches including sulfuric acid, phosphoric acid, hydrofluoric and hydrochloric acids; ammonium fluoride; ammonium, sodium and potassium hydroxides and hydrogen peroxide to mention a few. These materials create proper handling and waste storage problems. The current status of wet chemistry cleaning technologies is discussed by Hattori, "Trends in Wafer Cleaning Technology", Solid State Technol. Suppl., p.S7, May, 1995.
The use of dry ice snow flakes, formed by the expansion of liquid CO.sub.2 jet sprays, have also been used to clean spacecraft optical surfaces and semiconductor devices. These applications are discussed in M. M. Hills, "Carbon Dioxide Jet Spray Cleaning of Molecular Contaminants", J.Vac.Sci.Technol. A13(1), 30, January/February 1995 and R. Sherman et al., "Dry Surface Cleaning Using CO.sub.2 Snow", J.Vac.Sci.Technol.,B9(4),1970, July/August 1991. Although capable of removing organic films and particulates, CO.sub.2 jet sprays are ineffectual for removing submicron particulates (&lt;0.1 micron) to levels specified for future microcontamination-free manufacturing of wafers.
Other "dry" cleaning technologies for removing contaminants from semiconductor wafers include gas-phase cleaning which uses reactive gaseous radicals formed by the excitation of process gases. These processes suffer from the use of complex chemistries which can result in damaged surfaces or removal of substrate material when attempting to remove particulate contaminants.
In order to provide further background information so that the invention may be completely understood and appreciated in its proper context, reference may be made to a number of prior art patents as follows: U.S. Pat. No. 5,196,034 to Ono at al discloses a method for making fine ice particles from ultrapure water; forming an ice particle jet using gas under high pressure and directing a spray of ice particles against the surface of a semiconductor wafer. Ice particle jets formed in this fashion do not have sufficient velocity or size range necessary to dislodge fine submicron particulates below 0.1 micron. The process also suffers from introduction of contaminants via water, gas and transfer lines.
U.S. Pat. No. 5,148,823 to Bran discloses a cleaning system based on a high frequency megasonic process comprising a piezoelectric transducer and transmitter. A method is described which combines the rinse cycle with the cleaning cycle to reduce contamination by eliminating exposure of wafers to a solvent/air interface between cycles. This patent is representative of a class of disclosures relying upon ultrasound technology for removing particles from wafer surfaces; technologies known to consume large quantities of water and solvents requiring disposal in accordance with strict environmental regulations. Additionally, megasonic processing is an ex-situ, wet cleaning method, non-integratable into in-situ gas-phase or vacuum wafer preparation steps. As removal of "killer" particulates less than 0.1 micron becomes more crucial for higher yields of small devices, megasonic frequencies will have to increase beyond the state-of-the-art. Also megasonic cleaning containers must be tailored so as not to interfere with the cleaning power of the megasonic beam.
U.S. Pat. No. 5,089,441 to Moslehi discloses a method for low-temperature (650-800.degree. C.) in-situ dry cleaning process for removing native oxides grown on semiconductor surfaces exposed to wet chemical treatments or to ambient air during transport between process steps. Oxide removal occurs by exposing wafers at elevated temperatures to a dry-cleaning mixture of Germane (GeH.sub.4) and hydrogen gas. This method is similar to other gas phase cleaning processes which use reactive gases or gaseous radicals (with or without plasma assistance). Although cleaning organic residues and thin oxide films are demonstrated, these processes do not efficiently remove submicron particulates or trace metals without etch-pitting or damage to the wafer substrate. Furthermore many such dry cleaning methods use corrosive gases such as HF.
U.S. Pat. No. 4,806,171 to Whitlock et al discloses a method for removing particles from a substrate using a stream consisting of solid and gaseous carbon dioxide. This process removes particles which are highly resistant to removal by dry nitrogen streams blown across substrates. However, the solid/gas mixture of CO.sub.2 can not efficiently remove submicron particulates trapped in micron-sized etched trenches, etc. Additionally the process is limited to removal of particulate matter leaving other contaminants such as organic films partially intact Also, similar to U.S. Pat. No. 5,196,034, the velocity of the impacting CO.sub.2 solids is well below the threshold for inducing microshocks in the impacted material causing desorption and liftoff of ultrafine particulates, metallic and organic matter.
U.S. Pat. No. 5,232,563 to Warfield discloses an electrolytic bath configuration for removing a combination of contaminant materials from wafer surfaces including material lodged in surface recesses. A semiconductor wafer and inert conductive electrode are immersed in a bath consisting of water, an electrolyte such as HCL or HNO.sub.3 and a non-ionic surfactant (sulfonic acid). Passing a current through the cleaning cell using a voltage source generates oxygen bubbles at the wafer electrode surface thereby floating contaminants from the surface. A disadvantage of this configuration is that upon removal from the bath, wafer re-contamination occurs when breaking the liquid/air interface since the electrolytic bath surface has been enriched with previously removed contaminants. In common with other wet chemistry cleaning methods, multiple wafer cleaning will require frequent exchange of bath solutions ultimately consuming large quantities of water and chemicals. This ex-situ cleaning method is also likely to be slow, cumbersome and requires excessive non-automated wafer handling.
U.S. Pat. No. 5,151,135 to Magee et al discloses a method for sweeping short (80 nanoseconds or less), low energy pulses (0.1 to 0.3 J/cm.sup.2) of ultraviolet laser radiation(wavelength range 180-435 nm) across a substrate for removing chemical, metallic and particulate contaminants. Generally, laser cleaning techniques suffer from several disadvantages. High energy pulsed lasers risk substrate damage by introducing point defects, surface melting or annealing. Non-uniform or high energy homogenous pulses can actually "bake-on" contaminants due to excessive heat conduction. Low energy pulsed lasers produce photon impulses ineffectual for imparting enough momentum to dislodge particulates less than 0.1 micron strongly bonded to the surfaces by tenacious electrostatic forces. Furthermore, more than one pulse may be required to clean contaminants from surfaces irradiated by low energy laser pulses. Multiple passes will be required to clean heavily contaminated areas increasing the time required to clean surfaces. Introducing additional lasers to effect rapid cleaning only introduces complexity and additional expense for systems which are already costly. As disclosed, the usefulness of a pulsed UV laser as a primary cleaning step is unclear, since cited examples of cleaning started with surfaces previously cleaned by wet chemical means, subsequently describing the laser cleaning step as a "topping off" process.
Whatever the precise merits, features and advantages of the above cited references, none of them achieves the purposes of the present invention. Accordingly, it is desirable to provide an improved method and apparatus for removing thin film and particulate contaminants from semiconductor wafers simultaneously with higher efficiency, especially for particulate matter less than 0.1 micron in size. A principle object of the present invention is to remove contaminants from surfaces independent of their size, composition or morphology. Another object of the present invention is to introduce a technology for wafer cleaning which does not use hazardous chemicals or large quantities of highly purified, de-ionized and expensive water. A further object of the present invention is to provide an in situ method for cleaning wafers and wafer processing tools in vacuum, using a supersonic beam of microdroplets (clusters) distributed in size for removing submicron contaminants trapped in surface recesses, etched trenches or vias.